Re: [patch 8/8] x86/entry: Move irqflags tracing to do_int80_syscall_32()

Alexandre Chartre
2020-02-27T16:47:20Z

On 2/25/20 11:08 PM, Thomas Gleixner wrote:
> which cleans up the ASM maze.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> arch/x86/entry/common.c | 8 +++++++-
> arch/x86/entry/entry_32.S | 9 ++-------
> arch/x86/entry/entry_64_compat.S | 14 +++++---------
> 3 files changed, 14 insertions(+), 17 deletions(-)
>
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -333,6 +333,7 @@ void do_syscall_64_irqs_on(unsigned long
> {
> syscall_entry_fixups();
> do_syscall_64_irqs_on(nr, regs);
> + trace_hardirqs_on();
> }

trace_hardirqs_on() is already called through syscall_return_slowpath()
(from the previous patch):

do_syscall_64()
-> do_syscall_64_irqs_on()
-> syscall_return_slowpath()
-> trace_hardirqs_on()

> NOKPROBE_SYMBOL(do_syscall_64);
> #endif
> @@ -389,6 +390,7 @@ static __always_inline void do_syscall_3
> {
> syscall_entry_fixups();
> do_syscall_32_irqs_on(regs);
> + trace_hardirqs_on();
> }

Same here:

do_int80_syscall_32()
-> do_syscall_32_irqs_on()
-> syscall_return_slowpath()
-> trace_hardirqs_on()

> NOKPROBE_SYMBOL(do_int80_syscall_32);
>
> @@ -468,8 +470,12 @@ static __always_inline long do_fast_sysc
> /* Returns 0 to return using IRET or 1 to return using SYSEXIT/SYSRETL. */
> __visible notrace long do_fast_syscall_32(struct pt_regs *regs)
> {
> + long ret;
> +
> syscall_entry_fixups();
> - return do_fast_syscall_32_irqs_on(regs);
> + ret = do_fast_syscall_32_irqs_on(regs);
> + trace_hardirqs_on();
> + return ret;
> }
> NOKPROBE_SYMBOL(do_fast_syscall_32);

Same here:

do_fast_syscall_32()
-> do_fast_syscall_32_irqs_on()
-> do_syscall_32_irqs_on()
-> syscall_return_slowpath()
-> trace_hardirqs_on()

Except for one case (if the get_user() call is true in
do_fast_syscall_32_irqs_on()):

do_fast_syscall_32()
-> do_fast_syscall_32_irqs_on()
-> prepare_exit_to_usermode()

So we need to call trace_hardirqs_on() but only in that case:

static __always_inline long do_fast_syscall_32_irqs_on(struct pt_regs *regs)
{
...
if (
#ifdef CONFIG_X86_64
/*
* Micro-optimization: the pointer we're following is explicitly
* 32 bits, so it can't be out of range.
*/
__get_user(*(u32 *)&regs->bp,
(u32 __user __force *)(unsigned long)(u32)regs->sp)
#else
get_user(*(u32 *)&regs->bp,
(u32 __user __force *)(unsigned long)(u32)regs->sp)
#endif
) {

/* User code screwed up. */
local_irq_disable();
regs->ax = -EFAULT;
prepare_exit_to_usermode(regs);
trace_hardirqs_on(); <<<=== HERE
return 0; /* Keep it simple: use IRET. */
}
...
}

alex.

> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -811,8 +811,7 @@ SYM_CODE_START(ret_from_fork)
> /* When we fork, we trace the syscall return in the child, too. */
> movl %esp, %eax
> call syscall_return_slowpath
> - STACKLEAK_ERASE
> - jmp restore_all_switch_stack
> + jmp .Lsyscall_32_done
>
> /* kernel thread */
> 1: movl %edi, %eax
> @@ -968,8 +967,7 @@ SYM_FUNC_START(entry_SYSENTER_32)
>
> STACKLEAK_ERASE
>
> -/* Opportunistic SYSEXIT */
> - TRACE_IRQS_ON /* User mode traces as IRQs on. */
> + /* Opportunistic SYSEXIT */
>
> /*
> * Setup entry stack - we keep the pointer in %eax and do the
> @@ -1072,11 +1070,8 @@ SYM_FUNC_START(entry_INT80_32)
> movl %esp, %eax
> call do_int80_syscall_32
> .Lsyscall_32_done:
> -
> STACKLEAK_ERASE
>
> -restore_all:
> - TRACE_IRQS_IRET
> restore_all_switch_stack:
> SWITCH_TO_ENTRY_STACK
> CHECK_AND_APPLY_ESPFIX
> --- a/arch/x86/entry/entry_64_compat.S
> +++ b/arch/x86/entry/entry_64_compat.S
> @@ -132,8 +132,8 @@ SYM_FUNC_START(entry_SYSENTER_compat)
> movq %rsp, %rdi
> call do_fast_syscall_32
> /* XEN PV guests always use IRET path */
> - ALTERNATIVE "testl %eax, %eax; jz .Lsyscall_32_done", \
> - "jmp .Lsyscall_32_done", X86_FEATURE_XENPV
> + ALTERNATIVE "testl %eax, %eax; jz swapgs_restore_regs_and_return_to_usermode", \
> + "jmp swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
> jmp sysret32_from_system_call
>
> .Lsysenter_fix_flags:
> @@ -244,8 +244,8 @@ SYM_INNER_LABEL(entry_SYSCALL_compat_aft
> movq %rsp, %rdi
> call do_fast_syscall_32
> /* XEN PV guests always use IRET path */
> - ALTERNATIVE "testl %eax, %eax; jz .Lsyscall_32_done", \
> - "jmp .Lsyscall_32_done", X86_FEATURE_XENPV
> + ALTERNATIVE "testl %eax, %eax; jz swapgs_restore_regs_and_return_to_usermode", \
> + "jmp swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
>
> /* Opportunistic SYSRET */
> sysret32_from_system_call:
> @@ -254,7 +254,7 @@ SYM_INNER_LABEL(entry_SYSCALL_compat_aft
> * stack. So let's erase the thread stack right now.
> */
> STACKLEAK_ERASE
> - TRACE_IRQS_ON /* User mode traces as IRQs on. */
> +
> movq RBX(%rsp), %rbx /* pt_regs->rbx */
> movq RBP(%rsp), %rbp /* pt_regs->rbp */
> movq EFLAGS(%rsp), %r11 /* pt_regs->flags (in r11) */
> @@ -393,9 +393,5 @@ SYM_CODE_START(entry_INT80_compat)
>
> movq %rsp, %rdi
> call do_int80_syscall_32
> -.Lsyscall_32_done:
> -
> - /* Go back to user mode. */
> - TRACE_IRQS_ON
> jmp swapgs_restore_regs_and_return_to_usermode
> SYM_CODE_END(entry_INT80_compat)
>